With improvements to higher densities in semiconductor integrated circuits, photolithography for a submicron region is now an indispensable technique. Light of a shorter wavelength is necessary as the light to be used for exposure. Generally the ultraviolet ray (UV) called the G-line or I-line generated from a high pressure mercury lamp has been used but an excimer laser is currently used instead of the ultraviolet ray. For example, the excimer lasers of dimer consisting of a rare gas and a halogen gas, such as xenon chloride (XeCl), krypton fluoride (KrF), and argon fluoride (ArF), emit laser beams having wavelengths of 306 nm, 248 nm and 198 nm, respectively. Of these laser beams, the wavelength of 306 nm of the XeC excimer laser beam is not very different from the wavelength 365 nm of the I-line of a high pressure mercury lamp. Therefore, it was not profitable to use such a laser beam for exposure of a fine pattern and only the light source called deep-UV, having a wavelength of 248 nm and 198 nm emitted by a KrF excimer laser or an ArF excimer laser, is now expected to provide excellent results for exposure to form a fine pattern.
A photomask for UV or deep-UV corresponding to a light source for exposure of UV or deep-UV, is necessary, upon execution of photolithography. The photomask of this type has usually been fabricated by the following method. A thin film which absorbs light, such as chromium (Cr) or chromium oxide (CrO.sub.x), is formed on a mask substrate which transmits light. Next, the surface is then coated with a resist film and is patterned by electron beam lithography or UV light lithography. With this resist patterned used as the mask, a chromium film or chlomium oxide film is patterned and the desired photomask can thus be fabricated as described above.
Not only is the photomask fabrication method complicated but the accuracy of the photomask formed is governed by the accuracy of the resist pattern. Etching accuracy of the chlomium film or the chlomium oxide film and the photomask thus obtained is not suitable as the photomask for a submicron region.
In the UV and deep-UV wavelength band, absorption of light, as well as the environment gas, by an optical component and photomask substrate of the optical system for lithography such as a projection lens is generally distinctive. Moreover, a lattice defect called a color center is generated on these materials due to irradiation of UV and deep-UV. The lattice defect lowers the transmissivity of light through the materials. As a result, exposure intensity becomes small. Accordingly, selection of a material for a photomask substrate is particularly important.
Investigations for a generation mechanism and property of a color center have been made from various directions and these are described in detail, for example, in books by J. H. Schulman and W. D. Compton, "Color Centers in Solids", the MacMillian Co., (1962) and W. B. Fowler, "The Physics of Color Centers", (edited by W. B. Fowler) Academic Press (1968).